Scrambling system



Aug. 9, 1960 SCRAMBLING SYSTEM Filed Dec. 16, 1943 J. L. RUSSELL ETAL 6 Sheets-Sheet 2 4o 42 f RECEIVER BAND PASS VIDEO DISCRIMINATOR FILTER 1 AMPLIFIER [45 r 48\ f l RELAY L RELAY l 46 5| H.F. SWEEP INTEGRATOR 49 J COUNTER 44 55 J ,43 LI47- 5s RELAY TH I &

' AUDIO .F. [I L SWYEEP AMPLIFIER grvue/wbma JAMES L. RUSSELL ELVIN E. HERMAN A 1960 I J. RUSSELL EI'AL 7 2,948,779

SCRAMBLING SYSTEM Filed Dec. 16, 1943 6 Sheets-Sheet 3 I.LLE' E' 2 I5 I? 18 AUDIO i F 05c FREQUENCY HE swEEP AMPLIFIER 1 .MULTIPLIER GENERATOR LF 9w EP EEP' j E sw GENERATOR SELECTION 'NTEGRATOR ELECTRONIC36 ELECTRONIC 35 TEGRATOR v m SWITCH SWITCH UNBLANKING CONTROL I-AMPLIFIER MANUAL SWITCH TRANSMITTER FREQUENCY OSCILLATCRI s3 MODULATOR KEYER' -32 JWUW JAMES L. RUSSELL ELVIN E. HERMAN Aug. 9, 1960 Filed Dec. 16, 1943 J. L. RUSSELL ETAL SCRAMBLING SYSTEM 6 Sheets-Sheet 4 4o 47 RECEIVER BAND PASS VIDEO DISCRIMINATOR FILTER 4| AMPLIFIER v v RELAY RELAY 46 5| HF. SWEEP IL INTEGRATOR 49 J I r r56 ELECTRONIC ELECTRONIC s1 COUNTER SWITCH SWITCH I :L- RELAY IM. 44

C L.F. SWEEP f5; El I AUDIO AMPLIFIER glwucn'foz JAMES L. RUSSELL.

Aug. 9, 1960 Filed Dec. 16, 1945 J. L. RUSSELL ET AL 2,948,779 I SCRAMBLING SYSTEM I 6 Sheets-Sheet 5 lice-a5 I l 2 65 /66 57 AUDIO MULTW R MULTIVIBRATOR MULTIVIBRATOR DO AMPLIFIER' 'BRATO DIVIDER DIVIDER as V s 9 7o SWEEP SWEEP SWEEP GENERAToR GENERATOR GENERATOR SWEEP sEI EcToR TI I UNBLANKING CONTROL I -l I AMPLI IER I 1 I "v" "Vfivv ""I" TRANSMITT 4 FREQUENCY MODULATOR T gwuexwbw JAMES L. RUSSELL ELVIN E. HERMAN Aug. 9, 1960 -J.L.RUSSELL"ETAL 2,948,779

SCRAMBLING SYSTEM Filed Dec. 16, 1945 v s Sheets-Sheet s IE1|,3 E

4o 42 RECEIVER BAND PASS VIDEO DISCRIMINATOR FILTER 2 AMPLIFIER 9' WLTIVIBRATOR F MULTIVIBRATOR '1' MULT'V'BRATOR MULT'IPLIER MULTIPLIER i {r SWEEP SWEEP SWEEP GENERATOR GENERATOR GENERATOR v u? j I sweep SELECTOR V UNBLAN Y con-mom. v v

A I p r AUDIO AMPLIFIER 6 99 no, I I ZliVUC/HICOD JAMES L. RUSSELL ELVIN E. HERMAN SCRANIBLING SYSTEM James L. Russell and Elvin E. Herman, Washington, D.C.

Filed Dec. 16, 1943, Ser. No. 514,484

'1 Claim. (Cl. 179-15) v (Granted under Title 35, US. Code (1952), sec. 266) This invention relates to radio transmission of intelligence and is particularly directed to the problem of enabling such transmission to be effected in a practically undetectable manner.

It has hitherto been proposed to transmit intelligence by the spurt method wherein a message is recorded over an extended period and transmitted in a very-brief i period. With a suitable receiving'system the transmission is recorded at high speed and played back at low speed. Without special receiving Systems, the transmission simulates static bursts. In all hitherto suggested systems, such as application Ser. No. 444,276, filed May 23, 1942, for Short Wave Communication System filed by Robert M. Page, moving mechanical parts are employed for recording and consequently the'transmission period cannot be reduced below a period which is, for all practical purposes, severely limited. 'In addition, the mechanical complexity of the drive mechanism leads to constant maintenance demands.

It is the object of the invention to provide a'spurt transmission system without mechanical moving parts.

- It is a further object of the invention to provide a system for scrambled transmission.

Another object of the invention is to provide a spurt transmission system incorporating scrambled transmis- S1011.

The system of the present invention employs for a storage or recording element a charge pattern sustaining surface upon which the intelligence is recorded by means of a modulated electron beam moved across the surface. The stored signal is then removed veryrapidly and transmitted. By this system it is entirely practical to record on a single target surface for one minute and transmit the signal thereby accumulated in .03 second. Consequently, continuous operation may be obtained in- :asmuch as the incoming signal lost during transmission is so slight as to be negligible from a communications standpoint. The continuously operated system has the further advantage that the transmission repetition rate is so low as not to give the aural impression of a repetitive signal, and will normally be further masked by intervening static bursts.

The signal to be transmitted, which may be supplied .from a microphone circuit, is imposed as current or ve- .locity modulation on a beam of electrons which is swept over the target in a defined locus. In order to use the target area efliciently, such locus will normally comprise adjacent paths, such as parallel lines or a spiral trace substantially filling the target area.

The path is then rapidly retraced by the beam during the transmission period, and the target output signal produced by this wiping action modulates the transmitter. Inasmuch as the path is retraced directly in this type of operation, the transmission is either clear or inverted.

The method of the present invention, however, permits arbitrary scrambling not possible in previous'systems. In this respect, as will become manifest, the invention .is not restricted to spurt transmission, but may be em- 1C Patented Aug. 9, 1960 ployed in whatever application desired. Scrambling is obtained by sweeping the target area over a path different from that of the recording sweeps. This generates an output signal incorporating successively portions of difierent recording sweeps. The receiving system then applies the output signal to a target with a sweep pattern according with the transmission sweep pattern, and thereby regenerates the original recorded target voltage distribution pattern at the transmitter, which is next swept in accordance with the original transmitter recording sweep at a low velocity to reproduce the original signal.

For instance, the original signal may be modulated on: an electron beam swept by a saw-tooth wave in a ver-- tical direction and also swept at a low speed orthogonally thereto during the recording. Thereafter the target is: swept rapidly for transmission with a saw-tooth wave applied horizontally in the direction of the low speed recording sweep and slowly in the vertical direction of. the recording saw-tooth sweeps. The signal thereby generated and transmitted is scrambled so arbitrarily as to resist decoding without familiarity with the sweep systems employed. In this connection, it will be appreciated that a practically unlimited choice of arbitrary sweep patterns is available.

The invention will be further described with reference to the exemplary embodiments disclosed in the drawings, in which:

Fig. 1 shows in diagram a spurt transmitting system,

Fig. 2 shows in diagram a spurt receiving system,

Fig. 3 shows in diagram a scrambling system for spurt transmission,

Fig. 4 shows in diagram a scrambling system for spurt reception,

Fig. 5 shows in diagram a spurt transmission system,

Fig. 6 shows in diagram a spurt receiving system, and

Fig. 7 shows in diagram a sweep selector.

The transmitting system shown in Fig. 1 includes as an input channel microphone 1 and audio amplifier 2 which modulates the beam of cathode ray tube 3 through action of grid 4. Tube 3 contains the conventional cathode 5, first and second anodes 6 and 7 and deflection plate pairs 8 and 9. The gun elements are energized by the potential drops developed across divider 12. As will be understood, the beam positioning circuits are not shown. The tube also includes a charge pattern sustaining target '10 which may be a conventional fluorescent coating. The target output in the circuit shown is taken from signal plate 11 which is a conducting sheath covering the end of the tube and overlying the target area.

Such a storage device is further described in our copending application Ser. No. 514,485, filed December 16, 1943, for Signal Storage Device, now abandoned.-

The exemplary sweep locus established in the circuit of Fig. l is similar to the conventional television sweep comprising a set of adjacent parallel lines generated by a high frequency wave applied to one pair of deflection plates and a low frequency wave applied to the other pair of deflection plates for spacing the cycles of the high frequency wave.

The sweep system is controlled by a master low frequency oscillator 15 operating at the frequency of the recording sweep generator 16 which is synchronized by 15. Oscillator 15 may be any suitable sine wave generator, and its output is also fed to frequency multiplier-17 which in turn synchronizes the high frequency saw-tooth sweep 18 which functions during the transmission period.

The frequency ratio shift effected by multiplier 17 will be proportional to the ratio of the recording and transmission periods, which for instance maybe of the, order of 2000. In such case the high frequency sweep will generate 2000 cycles during the period of a single cycle the assumed example will have a division ratio of 2000.

A specific circuit which may be employed would include a condenser incrementally charged on each cycle supplied by generator 19 and discharged by a gas tube on attaining a predetermined voltage corresponding to the 2000th cycle. Manifestly frequency dividers such as multivibrators may precede the condenser circuit. The time constants of the discharge circuit of integrator 19 are selected to generate an output pulse of a duration equal to a single cycle of generator 16.

The saw-tooth sweep waves supplied by generators 16 and 18 are fed to a sweep selector 20 which consists of an electronic switch normally feeding the recording sweep from generator 16 to deflection plates 8 of tube 3. Selector 20 is controlled by the output from integrator 19 to switch the high frequency sweep 18 to deflection plates 8 during one cycle of the low frequency generator 16.

The phase of the synchronizing voltage supplied by frequency multiplier 17 to the high frequency sweep generator 18 is selected to effect synchronism of its output with that of generator 16. For this purpose the frequency multiplier may incorporate a variable phase shifting network. In the case described, therefore, the beginning of each cycle of generator 16 will coincide in time with the beginning of each 2000th sweep of generator 18. The operation of sweep selector 20 is to establish at its successive switching periods a sweep voltage of the same relative phase from generators 16 and 18.

The sweep voltage for deflection plates 9 is applied by integrator 25 which responds to the flyback strokes of the sweep output from selector 20. This integrator is designed to have a very short discharge time, and a counting ratio of 2000 in the specific example. This produces an incremental sweep orthogonal to the sawtooth sweeps to space the lines across the target area.

If desired, injected tripping voltages for integrator 25 may be derived from integrator 19 at the beginning of the transmission sweep, which coincides with the end of the recording sweep, and at the end of the transmission sweep, by suitable circuit components operating to differentiate the impulse from integrator 19.

The output of integrator 25 is of the same magnitude for each sweep cycle, whether supplied by generator 16 or generator '18. Upon inauguration of operation, it will be seen that the low frequency saw-tooth wave from generator 16 is supplied to both integrators 19 and 25, and the integrators will therefore run in synchronism.

The switching operation of sweep selector 20 effects successive sweeps over the same identical locus on the target area at velocities difiering by a factor of 2000 in the specific example.

The electron gun of tube 3 is normally blanked by the bias of grid 4 on divider 12, and the tube is unblanked on each sweep by control 26. This circuit may comprise differentiating components responsive to the saw-tooth voltages. This has the advantage of increasing the beam current on the high velocity take-E sweeps.

Assuming that the input channel supplies frequencies of the range from 250 to 2500 cycles, the output channel fed by signal plate 1 1 will include frequencies from 500 kc. to me. and consequently output amplifier 27 is designed to operate within these limits. Although the input channel could be blanked during the transmission period, in the embodiment shown the incoming signal at the time of transmission produces no deleterious effect as 4 it is not within the frequency range of output amplifier 27.

The transmitter 30 shown in Fig. 1 is frequency modulated by component 31 which receives the output signal from amplifier 27. The transmitter is keyed at 32 by the signal generated during the fast sweep period by integrator 19.

The keying signal is received by the transmitter directly from integrator 19, whereas the storage tube output signal which modulates the transmitter is delayed in passage through amplifier 27. The preliminary operating time of the transmitter before receiving the stored signal is employed in this embodiment to transmit a keying signal used in the operation of the receiving apparatus to be described. For this purpose oscillator 33 is provided. This operates continuously and therefore immediately upon inauguration of transmitter operation a modulated signal is radiated. Preferably the modulating frequency is outside the frequency range of the storage to the output signal. Were the unmodulated carrier employed for keying the receiver, the latter might be subject to keying by noise impulses, whereas in the system described, this is not the case.

In order to obtain the requisite transmitter operation period which includes the preliminary keying signal, keyer 32 will have a slight holdover beyond the end of the impulse from integrator 19.

Signals developed across the storage tube signal plate load resistor during the recording period are rejected, being below the pass band of output amplifier 27.

Having described the transmitter operation, it is apparent that a salient advantage over previous mechanical systems lies in the instantaneous shift which may be elfected in the scanning speed to effect transmission. Mechanical systems are not adapted to continuous operation because the input channel signal is lost during the speed-up period of the record base which greatly exceeds the actual transmission period.

The receiving system shown in Fig. 2 includes a receiver 40 which is responsive to the frequency modulated signal from transmitter 30, and supplies a demodulated output.

Band pass filter 4-1 is provided to select the transmitted frequency component from oscillator 33 in order to key the receiver storage tube sweeps in proper relation to the transmitted signal. The receiver output is also fed through video amplifier 42 to control grid 43 of storage tube 44. This tube is in all respects similar to tube 3 of the transmitter system, and is employed to record the transmitted signal at a very rapid rate and then reproduce the signal supplied to the transmitter input channel at its normal rate.

For this purpose the keying signal is applied to relay 45 of the unbalanced multivibrator type to generate an output pulse having a duration equal to the period of transmitter operation. This pulse is slightly delayed in its inauguration for the purpose explained below.

The relay pulse is fed to high frequency sweep 46 which is thereby thrown into operation. This may be effected by a switching tube operative to supply anode potential to the sweep generator during duration of the relay impulse. The sweep voltage is applied to deflection plates 47 and also generates an incremental orthogonal sweep in integrator 51 which is applied to plates 58. The function of integrator 51 is similar to that of integrator 25 in the transmitter system.

The frequency of saw-tooth sweep generator 46 is preferably equal to that of the high frequency sweep 18 of the transmitter system. However, in case efiicient communication but not fidelity is required, the sweeps need not be synchronized as this results only in additional loss of retrace time on the receiver recording sweeps, which for such purpose is entirely inconsequential. When the sweeps are synchronized, however, the transmitter signals corresponding to the flyback time of the transmitter saw tooth sweeps coincide with the corresponding portion of the receiver sweep and consequently none of the transmitted signal is lost.

In Fig. 2 the inauguration of the first cycle from sweep 46 is synchronized with the delivery to grid 43 of the first signal from the transmitter storage tube. For effecting this coincidence the transmission delay present ,in video amplifier 42 is taken into account, as well as the delay in filter 41 and the response delays of relay 45 and sweep 46.

Video amplifier 47 is normally inoperative so as to supply no signal to grid 43. It is gated by relay 48 which holds it in operation for the period of transmission, after which it again becomes inoperative for the duration of the reproduction period of the receiving system.

Relay 45 maintains the high frequency sweep in operation during the transmission period which will comprise 2000 cycles in the system described. Counter circuit 49 receives the output of the high frequency sweep and may be used to supply a pulse at the end of the transmission period to terminate operation of relay 45 and sweep 46. Such operation has the advantage over sole reliance on the time duration of relay 45 as this may vary, in practical application, with the power supply voltage and other factors.

The primary function of counter 49 is to effect operation of low frequency sweep 50 during the reproduction period and relay 55, responsive to the output of counter 49, therefore has an operating time equal to this period. Consequently the low frequency sweep 50 is immediately thrown into operation at the end of the recording operation and begins the low velocity reproduction sweep period.

It will be understood that the saw-tooth sweep voltages generate the incremental orthogonal sweeps by action of integrator 51. Such integrator acts as a counter in that it begins a new cycle at the end of the high frequency sweeps and in the reproduction period causes the beam to retrace the recording sweep locus.

The reproduction is accomplished by audio amplifier 52 and reproducer 53 fed from signal plate 54. This system is not rendered inoperative during recording due to the short time period thereof and the fact that the frequencies present are well above the transmission band of such system.

Where the system is to be employed for the communication of short messages of a duration of one or a few recording-transmission cycles, the transmitter control circuit feeding keyer 32 may be provided with a manually controlled switch 34 to transmit only during the desired period. The receiving system is responsive to any number of transmission cycles and requires no alteration or additional controls for such operation.

The circuits shown in Figs. 3 and 4 comprise an exemplary system for scrambled spurt transmission. It will be appreciated that the scrambling features are applicable to other types of transmission.

The transmitting system of Fig. 3 difiers from that of Fig. 1 in that the signal from the input channel is laid on the storing target over a first sweep locus and thereafter the target voltage distribution pattern is taken off along a difierent sweep locus, generating a scrambled signal, and transmitted. The receiving system of Fig. 4 is provided with recording sweep similar to the second mentioned transmitter sweep which upon receipt of the transmitted signal establishes upon the storing target in the receiver a voltage distribution pattern identical with that recorded on the transmitter target. This pattern is then scanned with a reproducing sweep similar to the original transmitter recording sweep to reproduce the transmitter input channel signal. In accordance with the principles of spurt transmission already discussed, the transmitter take-off sweep and the receiver recording sweep are performed at very high velocity relative to that of the transmitter recording sweep and the receiver producing sweep.

Specifically in the circuits shown, the input channel signal is laid on the transmitter target along a multiplicity of substantially parallel lines, and the target is then swept for transmission along another set of sweep lines orthogonal to the first set. The receiver sweeps are similarly related.

The circuit of Fig. 3 is functionally similar to that of Fig. 1, but additionally includes electronic switches 35 and 36. Switch 36 normally feed the output of sweep selector 20 to deflection plates 8 of tube 3. Switch 35 normally feeds the output of integrator 25 to plates 9. During the recording period, therefore, the operation is identical with that of the previously described system.

During transmission, however, the control impulse from integrator 19, which causes sweep selector 20 to supply the high frequency sweep from generator 18, is also applied to switches 35 and 36 to apply the high frequency sweep to plates 9 and the integrator output to plates 8. Consequently, the input channel signal is arbitrarily scrambled during transmission.

The receiver shown in Fig. 4 also includes additional switching means comprising electronic switches 56 and 57. Switch 57 normally feeds the saw-tooth sweep voltage to deflection plates 58, while switch 56 normally feeds the integrated sweep voltage from 51 to plates 59. The recording sweep is accordingly effected by the generation of a series of horizontal lines which cause a voltage distribution pattern on target area identical with that imposed on the transmitter target area.

On the reproduction cycle, the control signal from relay 55 is applied to switches 56 and 57 to apply the low frequency saw-tooth sweep to plates 59 and the integrated sweep voltage to plates 58 to trace the pattern in the order in which it was originally applied to the transmitter target. This therefore reproduces the input channel signal, which is supplied to reproducer 53 by the audio amplifier 52.

The transmission system shown in Figs. 5, 6, and 7 is in some respects of simplified design in comparison with those previously described. This circuit effects spurt transmission in the manner provided for in the system of Figs. 1 and 2.

In Fig. 5, storage tube 3 with the input channel from microphone 1 and the signal plate output channel for modulating transmitter 30 is in all respects as described previously.

The sweep generating system employs three generators, the frequency ratio of the high frequency generator to. the medium frequency generator being the same as the:

frequency ratio of the medium frequency generator to the low frequency generator. Therefore, the long period: recording sweep may be carried out with the low fre-- quency generator supplying a linear sweep and the medium frequency generator supplying the saw-tooth orthogonal thereto and the high velocity transmission sweep may then be effected during a single cycle of the medium frequency sweep, which supplies the linear sweep, and the high frequency used to supply the saw-tooth sweep. The switching circuit for establishing the correct sweep voltages on the deflection plates is shown in Fig. 7. This circuit also supplies a keying voltage for transmitter 30.

The sweep system comprises multivibrators 65, 66, and 67. Multivibnator establishes the control frequency, and the output frequencies of 66 and 67 are controlled thereby. Where the frequency division ratio of multivib-rators 66 and 67 is large, these may include multiple stages to eifect reliable frequency reduction.

The multivibrator outputs are employed tokey sarwtooth wave generators 68, 69, and 70. The latter may be of the gas discharge tube type.

It will be understood that the operating conditions of the sweep components are such that the initiation of each sweep cycle by generator 70 coincides with the initiation of a sweep cycle of generator 69, and that the initiation of each cycle by generator 69 coincides with the initiation of a sweep cycle by generator 68.

The storage system operates to effect transmission during the forward stroke of generator 69 immediately fol lowing the flyback stroke of low frequency generator 70. For this purpose generator 70 supplies a control voltage to sweep selector 71 for inaugurating a transmission cycle. This will supply the high frequency sweep generator voltage to vertical deflection plates 9, and medium frequency sweep voltage to horizontal deflection plates 8. The series of parallel lines thereby swept on the target constitutes the sweep locus.

The flyback stroke of the medium frequency sweep generator 69 is then applied to the sweep selector to terminate the transmission sweep and inaugurate the recording sweep by connecting medium frequency sweep generator 69 to vertical deflection plates 9 and low frequency sweep generator 70 to horizontal deflection plates 8. At the end of the recording period the transmission sweep is again inaugurated by the flyback stroke of low frequency generator. It is therefore apparent that during the transmission period, occupying a single cycle of the medium frequency generator, the low frequency sweep voltage passes through a fraction of its cycle proportional to the frequency ratio of the low frequency and medium fre quency generators. This portion of the recording sweep locus, representing a single line, is therefore not imposed on the target because during this period the beam is traversing the full sweep locus, including this line, for generating the transmitter modulating signal. However, since the first line of the transmission sweep traverses this vacant target area, this preliminary interval is available for the transmission of the receiver keying signal.

The transmitter, for this purpose, is modulated with the signal from oscillator 33.

An exemplary switching circuit suitable for use as sweep selector 71 is shown in Fig. 7. The switching voltages are generated by tubes 75, 76 and their associated compo nents. The voltages control the transmission of the sweep generator voltages through tubes 77 and 78 to the vertical deflection plates and through tubes 79 and 80 to the horizontal deflection plates.

Tubes 75 and 76 are connected as an unbalanced multivibrator wherein tube 76 is normally conducting, its grid being returned to positive supply 81 through resistor 82. Tube 75 is normally biased to cutoff or to a low level across common cathode resistor 83. Consequently anode 84 of tube 75 is at substantially the potential of supply 81, whereas due to the voltage drop across resistor 87 anode 85 of tube 76 is at a low potential.

The output from low frequency generator 70 is applied at terminal 89 as a control voltage which on the negative flyback stroke drives tube 76 to cut-off, raising the potential of anode 85 which is coupled through oapacity 91 to the grid of tube 75, and thereby driving the latter into conduction. This effects a drop in potential at anode 84.

Due to the time constants of the circuit, this condition is maintained until the recording flyback stroke of the medium frequency sweep generator 69 which is applied as a control voltage at terminal 90. The negative flybuck voltage shift drives the grid of tube 75 toward cut-off and the resultant potential rise at anode 84, which is coupled to the grid of tube 76, drives the latter back into conduction.

Since the flyback stroke of low frequency generator 70 marks substantially the start of the forward stroke thereof, which is synchronized with the beginning of a forward stroke of generator 69, the conduction period of tube 75 begins with said forward stroke and terminates therewith also on the next flyback of generator 69.

The resultant voltage variations at the anodes of tubes 75 and 76 key the grids of tubes 77, 78, 79, and 802 Thus during the recording period when tube 76 is conducting and anode 34-. of tube 75 is at high potential, the connection of the latter through capacities 95 and 96 to the grids of tubes 77 and 79 maintains these tubes conductive. of tube 76 which is coupled through capacitors 97 and 98 to the grids of tubes 78 and 80, holds the latter tubes nonconductive. The other condition of the multivibrator circuit effects conduction of tubes 78 and 80, and blocking of tubes 77 and 79.

The output signal of medium frequency sweep generator 69 is applied at terminal 99 from which it is coupled to the grids of tubes 77 and 80. High frequency sweep 68 is fed to the grid of tube 78 at terminal 100, and the low frequency output of generator 70 is supplied to the grid of tube 79 from terminal 101.

The sweep selector output voltages are taken from terminals 102 and 103. Terminal 162 feeds the horizontal deflection plates, and 103 feeds the vertical deflection plates. Terminal 102 is fed from two anodes of tubes 79 and 80, and terminal 103 is fed from the anodes of tubes 77 and 78.

Consequently during the recording period the forward stroke of low frequency generator 70 is supplied to horizontal deflection plates 8 through tube 79, while medium frequency generator 69 supplies a vertical sweep to plates 9 through tube 77. For the transmission sweep, medium frequency generator 69 is applied to the horizontal deflection plates 8 through tube 80 and high frequency generator feed the vertical deflection plates through tube 78.

The transmitter is keyed from terminal 104 which supplies a positive pulse during the transmission sweep period to effect operation of the transmitter.

The receiving system shown in Fig. 6 includes a receiver and discriminator 40 which feeds: a band pass filter 41 and video amplifier 42. The video amplifier 42 is controlled by gating circuit which is responsive to the frequency component generated by oscillator 33 in the transmitted signal. The gating circuit, which may be a detector and amplifier, supplies an output impulse during the transmission period, which recurs at the frequency of transmitter sweep generator 70.

The gating circuit impulse is applied to video amplifier 4-2 to maintain operation thereof during the transmission period, as by application as a positive potential to a grid of a normally blocked tube. The gating circuit impulse is also applied as a synchronizing control voltage to the sweep generation system.

The latter includes three multivibrators 111, 112, and 113 operating on the frequencies of multivibrators 67, 66, and 65 respectively. Multivibrator 111, operating at the low frequency, is synchronized with transmitter multivibrator 67 by injection of a control voltage derived from gating circuit 110.

Multivibrator 112 is operated as a frequency multiplier synchronized from 1 11, and may comprise a plurality of stages where the frequency ratio is large. Multivibrator 113 is similar to 112.

The sweep generators 114, 115, 116 are similar to sweep generators 68, 69, and 70 of the transmitter.

The vertical deflection signals are fed to unblanking control circuit 26 as storage tube 44 is normally biased beyond cut-off.

Sweep selector 117 operates similarly to sweep selector 71 of the transmitter and on the receipt of the transmitted signal, coincident with the fiyback stroke of low frequency multivibrator 111, applies the high frequency saw-tooth wave of generator 116 across the vertical deflection plates 58 and a single stroke from generator to the horizontal deflection plates 47. In the receiving system this constitutes the recording sweep, the output of video amplifier 4-2 being applied to grid 43 of tube 44: for the purpose of modulating the beam with the transmitted si nal.

On the succeeding flyback stroke of generator 115 which, because of the synchronous relations described correspondingly, the low potential at anode 85 above, coincides with the termination of the transmitter signal, the sweep selector applies the low frequency generator output to the horizontal deflection plates 47 and the medium frequency signal to deflection plates 58. The video amplifier becomes inoperative under termination of the transmitter signal. The low frequency wiping sweep generates an audio output signal from plate 54 which is fed to output amplifier 52. and the original transmitter input channel signal is reproduced at 53.

The circuit of Fig. 7 may also be employed as sweep selector 117, in which case generator 114 will be connected to terminal 101 and to terminal 8 9 as a control voltage, generator 115 to terminal 99 and to terminal 90 as a control voltage, and generator 116 to terminal 100. The signal from terminal 102 is applied across the horizontal deflection plates 47, and the signal from terminal 103 is applied across vertical deflection plates 58.

It will be understood that the embodiments disclosed are exemplary and that the scope of the invention is to be determined with reference to the appended claim.

The invention descnibed herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

We claim:

In combination, a cathode ray tube including an electron gun for generating an electron beam, a target receiving said beam, sweep generator means comprising a low frequency saw-tooth generator, a medium frequency saw-tooth generator operable at a frequency which is a selected multiple of said low frequency, and a high frequency saw-tooth generator operable at a frequency which is the said selected multiple of the medium frequency, beam deflection means, means for applying the low and medium frequency saw-tooth waves to the beam deflection means to sweep the beam over a locus of adjacent parallel paths defining an area on the target, means becoming operative on the completion of said locus to apply the medium and high frequency saw-tooth waves to the beam deflection means to retrace said target area with a second locus of parallel paths orthogonal to the first paths at a higher velocity, beam modulation means operative to apply an intelligence signal to the beam during one sweep locus generation, and output circuit means responsive to the progressive voltage shift of the target during the other sweep locus generation.

References Cited in the file of this patent UNITED STATES PATENTS 2,146,876 Zworykin Feb. 14, 1939 2,175,573 Schroter Oct. 10, 1939 2,273,172 Beers Feb. 17, 1942 2,277,516 Henrotcau Mar. 24, 1942 2,281,405 Barrish Apr. 28, 1942 2,292,045 Burnett Aug. 4, 1942 2,321,611 Moynihan June 15, 1943 2,395,744 Kent Feb. 26, 1946 2,422,295 Eaton June 17, 1947 2,524,837 Russell et al Oct. 10, 1950 FOREIGN PATENTS 613,925 Germany May 2-9, 1935 

